[TUHS] Did realloc ever zero the new memory?

scj at yaccman.com scj at yaccman.com
Sat Sep 12 02:44:56 AEST 2015

Maybe I can clear up some of the confusion.  I too believe that malloc and
realloc never zeroed the newly added memory.  However, at some point
someone realized that when Unix added a page to the memory available to a
process, it didn't clear that page.  Somebody wrote a small program that
would, from time to time, malloc a chunk of memory and look for
interesting things in it (e.g., ASCII strings).  It was rather effective,
and soon Unix was changed so that any new pages were cleared before they
were added to the process.

Many mallocs tried to allocate a page if you asked for that size memory
(it depended on whether the control info was in the same arena as the
memory blocks or not).  So if your program used such a malloc and you
allocated full pages at a time, you might have silently seen malloc turn
into calloc, albeit in a completely non-portable way.  I suspect realloc
worked similarly.


> I've never heard before of a belief that the new memory belonging
> to realloc is always cleared, except in conjunction with the utterly-
> mistaken belief that that's true of malloc as well.  I don't think it
> was ever promised to be true, though it was probably true by accident
> just often enough (just as often as with malloc) to fool the careless.
> Norman Wilson
> Toronto ON
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